In order to install or service an HVAC system, HVAC service personnel must measure the HVAC system refrigerant pressure and from the refrigerant pressure determine the saturated vapor equivalent temperature for the particular refrigerant used in the HVAC system being installed or serviced. Conventionally, the refrigerant pressure is measured with an analog pressure gauge and the saturated vapor equivalent temperature for the refrigerant is determined from the measured refrigerant pressure either by the use of lookup tables for by the use of correlated scales on the face of the analog pressure gauge.
A typical analog pressure gauge is mechanical and contains a bourdon tube. The bourdon tube is a mechanical transducer. The bourdon tube is a curved and partially flattened tube that tends to straighten out in proportion to internal pressure within the bourdon tube. The bourdon tube is connected to a series of watch-like gears that rotate the indicating needle located on the front of the analog pressure gauge. When the bourdon tube of the analog pressure gauge is connected to the refrigerant line, the refrigerant pressure within the bourdon tube causes the bourdon tube to straighten out so that the needle, mechanically connected to the bourdon tube through the gears, indicates the refrigerant pressure on the face of the analog pressure gauge.
A conventional analog refrigerant pressure gauge 11 is shown in FIG. 1. When such an analog pressure gauge 11 is used to service an HVAC system, the main analog scale 13 of the analog pressure gauge 11 indicates pressure in PSI (pounds per square inch) or KPa (metric Kilo-Pascals). Additional inner analog scales (circular bands of numbers) 15 are also printed on the face of the analog pressure gauge 11. The additional inner scales 15 indicate the saturated vapor equivalent temperature for different refrigerants at the measured refrigerant pressure. Because of the limited space on the analog pressure gauge face, only two or three different inner scales for different refrigerants can fit onto the face of any one analog pressure gauge. FIG. 1 shows a typical prior art analog pressure gauge with only two saturated vapor equivalent temperature scales (refrigerants R-22 and R-134a) on its face.
The saturated vapor equivalent temperature scale of the analog pressure gauge is of importance to the HVAC technician because the saturated vapor equivalent temperature indicated on the analog pressure gauge for a particular refrigerant is used to ascertain the temperatures in parts of the system during charging, servicing, or monitoring. The correct saturated vapor equivalent temperature scale on the face of the analog pressure gauge must be matched to the type of refrigerant in the system. If the analog pressure gauge does not have a saturated vapor equivalent temperature scale that matches the refrigerant in the system being serviced, the technician must consult a table that converts the pressure read on the analog pressure gauge to the saturated vapor equivalent temperature. Because HVAC systems respond very slowly and because the pressure and the saturated vapor equivalent temperature must be continuously monitored during refrigerant charging, the use of a lookup chart is inconvenient, time consuming, and error prone.